At present, in the field of wireless communication technologies, e.g., the 2nd Generation (2G) and the 3rd Generation (3G) of mobile communication technologies, a wireless communication system is deployed typically with a macro base station and also a supplementary indoor Distributed Antenna System (DAS) and repeaters to implement the coverage by radio signals. For example, a Global System for Mobile Communications (GSM) achieves its radio coverage typically with Base Station Controllers (BSCs) and Base Transceiver Stations (BTSs); while the radio coverage is achieved typically with Base Band Units (BBUs) and Radio Remote Units (RRUs) in the 3G field.
There are generally the following three structures of a currently common DAS system:
As shown in FIG. 1, there is a schematic structural diagram of a first DAS system based upon BSCs, BTSs and an RF conditioning. The system is an analogue radio remote system, and in the system shown in FIG. 1, a plurality of BTSs output RF analogue signals and input the RF analogue signals into a DAS head end over optic fiber links. The input RF analogue signals at different carrier frequencies are combined, allocated, coupled and superimposed by the RF conditioning and converted by electro-optical converters into multiple RF coupled optical signals, which are finally output through remote antennas. In this case, signals output over the remote antennas can result from coupling and superimposition of any one or more of the signals output by the BTSs, that is, multimode signals are output.
In the DAS system shown in FIG. 1, since the analogue signals are transmitted over the optical fibers, and the analogue radio frequency signal transmission and the optical signal transmission may interfere with each other, the degradation of the system performance is caused; and also since the larger the optical fiber length is, the higher a noise coefficient of the system will be, the remote distance of the RF remote ends is limited in order to guarantee the system performance.
As shown in FIG. 2, there is a schematic structural diagram of a second DAS system based upon BSCs, a BBU and remote RF modules. In this system, the multiple BSCs and a packet switched network output digital baseband signals directly to the BBU over digital baseband links; the BBU routes and allocates the digital baseband signals, and outputs the routed digital baseband signals to the remote RF modules over digital optical links; and the remote RF modules modulate the digital baseband signals onto corresponding carrier frequencies and then output them through remote radio frequency ends. In this system, the digital baseband signals are transmitted over the optical fibers, thus lowering mutual interference between signal transmission and optical transmission and guaranteeing the system performance. However the BBU can only route the input digital baseband signals simply without any superimposition of those signals, so signals output by the remote radio frequency ends are merely single-mode signals.
As shown in FIG. 3, there is a schematic structural diagram of a third DAS system based upon BTSs, BBUs, low power-consumption RF modules and remote RF modules. This system is also an analogue radio frequency remote system in a similar architecture to the first DAS system, where the BTSs output RF analogue signals at different carrier frequencies, and the RF analogue signals are coupled and superimposed by an RF conditioning and converted by electro-optical converters into multiple RF coupled optical signals which are finally output through remote antennas, to thereby implement the coverage by multimode signals. At the same time, in order to support coupling and superimposition of baseband signals of the BBUs, the multiple low power-consumption RF modules are added to the system. The RF modules modulate the baseband signals output by the BBUs onto corresponding RF carriers, and the RF conditioning performs RF coupling and superimposition on the RF carriers to obtain multiple RF coupled optical signals, so that signals output over the multiple remote antennas can result from coupling and superimposition of any one or more of the signals of the BTSs and the BBUs, to thereby implement the coverage by multimode signals.
The third DAS system also suffers from the same problem of the system performance as the first DAS system, and moreover, the baseband signal superimposition technology is adopted in the third DAS system, but it is performed by up-converting and then RF-coupling the baseband signals, and consequently it is necessary to deploy the multiple RF modules in the system, thus making it difficult and costly to deploy the system.
Also, since the remote antennas can be installed only at a site where optical fiber lines are deployed, the existing DAS system has to be installed with a large-capacity backhaul network of optical fibers or the like, thus making it difficult to deploy the network, and moreover, the existing DAS system usually has to be extended in capacity by adding a full set of equipments including a BTS, a BSC, an MSC, etc., thus making it difficult and costly to extend the capacity.